Interior Minister Bajram Rexhepi said Kosovar authorities were forced to move after the European Union's rule of law mission, known as Eulex, failed to enforce the ban. He said some 40 policemen are in control of one checkpoint, while a second group is engaged in a tense stand off with local Serbs blocking access to another border crossing.

Both crossings were set on fire by rioting Serbs on the eve of Kosovo's secession and have since been loosely manned by the 3,000-strong EU mission.

Eulex spokesman Nicholas Hawton said the force was not part of the operation.

Nato peacekeepers have been deployed to the area to deal with any violent threats and their top commander Major General Erhard Buehler has engaged in talks with Serb demonstrators to defuse the situation, a Nato official said.

Mr Rexhepi said the situation was tense but pledged police would not "endanger the lives of the citizen".

Kosovo authorities have criticised Eulex for not moving against Serb warlords in Kosovo's north and of being biased against former ethnic Albanian rebels that fought against Serbia during the 1998-99 war.

Eulex has launched a series of investigations into alleged war crimes committed by the former rebels as well as embezzlement of public funds by the former guerrillas turned political leaders. The overnight action could undermine progress made between Kosovo and Serbia in EU-sponsored talks aimed at resolving practical issues, such as travel documents.

Kosovo has been recognised by 77 nations. But Serbia has successfully blocked its membership in the United Nations, where many countries share its rejection of unilateral declarations of independence by separatist regions.